1. Field of Invention
The invention relates to a fuel cell system, and more particularly, to a fuel cell system which serves to circulate cathode off-gas to a fuel cell so as to humidify an electrolyte membrane.
2. Description of Related Art
In the case where the fuel cell is of a polymer electrolyte membrane type, in which an electrolyte membrane is interposed between an anode and a cathode, the electrolyte membrane has to be sufficiently humidified so as to improve the proton conductivity. Published Japanese Translation of PCT application, JP-T-8-500931 discloses the technology in which a vapor-containing cathode off-gas that has been generated through an electrochemical reaction in the fuel cell is partially circulated together with an oxygen-containing gas (air) so as to easily humidify the electrolyte membrane. Japanese Patent Application Publication No. 2004-22487A may also be regarded as being relevant to the invention.
There may be the case where water contained in the cathode off-gas freezes the valve for controlling circulation of the cathode off-gas when the system is stopped under a low-temperature environment. The frozen valve in the opened state fails to appropriately control the circulation amount of the cathode off-gas after start-up of the system. This may supply excessive amount of the vapor- or nitrogen-containing cathode off-gas to the fuel cell, resulting in flooding. The partial pressure of the oxygen in the oxygen-containing gas is reduced to deteriorate the power generation efficiency.